Imagine a field of flowers; a field of flowers that are pollinated by basically three different pollinators. one pollinator, bees, attracted to short plants. another, butterflies, prefer plants of medium height and a third pollinator, birds, visits only the tallest plants. pesticide spraying kills many insect pests in the field of flowers but also wipes out the butterfly and bee populations. over time, only birds pollinate tall flowers and eventually the flower gene pool favors taller flowers. this is also an example of selection.